1. Field of the Invention
The present invention relates to a display element and a display device. The present invention especially relates to the digital-driven display element, and the digital-driven display device which are provided with a plurality of pixel circuits.
2. Description of the Related Art
An organic electroluminescent display device (hereinafter also called “an organic EL display device” or “an organic EL panel”) receives attention as a new flat display device. The organic EL display device is presumed to overtake a liquid crystal display device, which is currently widely used, in the near future, and is in the process of fierce development competition toward actual use.
There are mainly two types of driving methods for the organic EL display device. One is an analog driving method, and the other is a digital driving method. In the analog driving method, an electric current an amount of which is in accordance with data voltage is supplied to each organic EL element, so that the organic EL element emits light with luminance in accordance with the data voltage. Various methods are proposed as for the digital driving method. Taking a case of a time gradation method, for example, since a pulse current having a duty ratio in accordance with data voltage is supplied to each organic EL element, the organic EL element emits light for a time period in accordance with the data voltage to express multiple-gradation.
In a subfield driving method of the time gradation method, a single field (frame) period, which is a display period of a single frame, is divided into a plurality of subfield (sub-frame) periods. Controlling the on/off of lighting in each subfield period makes the organic EL element emit light for a time period in accordance with data voltage. In this case, since the same amount of electric current is supplied to the organic light emitting elements, the organic light emitting elements emit light with the same luminance, but difference in the length of lighting time expresses gradation. The light emission period of each subfield has length of 2 to the nth power (n=0, 1, 2, . . . N−1). The on/off of the light emission period set at length of, for example, 1, 2, 4, 8, 16, 32, 64, and 128 expresses 256 levels of gray.
In the organic EL panel adopting the time gradation method described above, it is preferable to operate a driving transistor, which drives the organic light emitting element at a constant current, in a linear region for the purpose of reducing the effect of variations in the transistors.
See, for example, Japanese Patent Laid-Open Publication No. H10(1998)-312173.
According to the digital driving method, however, when the driving transistor is operated in the linear region, there is a problem that variation in electric current due to variation in temperature and degradation with time is large, as compared with the analog driving method in which the driving transistor is operated in a saturation region. The large variation in electric current, which is supplied to the organic light emitting elements, causes dispersion of the luminance of the organic light emitting elements, so that there is the fear of the degradation of display quality.